1. Field of the Invention
The present invention relates to a system and a method for cooling superconductors.
2. Description of Related Art
A superconducting electromagnet device using a superconducting coil put in a vacuum insulation container is known (refer to Japanese Laid-Open Patent Application JP-H07-142237 and Japanese Laid-Open Patent Application JP-2004-111581, for example). Such a superconducting coil is cooled by a superconducting coil cooling unit and goes into a superconducting state under an extremely-low temperature condition. It is possible to generate a strong magnetic field by supplying a current to the superconducting coil in the superconducting state. Here, a member for externally supplying the current to the superconducting coil is generally called a “current lead”. That is to say, a drive current is supplied to the superconducting coil through the current lead when driving the superconducting coil.
In such a superconducting electromagnet device, heat intrusion caused by Joule heating at the current lead due to the current at the time of power distribution and heat intrusion from the outside of the vacuum insulation container to the superconducting coil through the current lead occur. In order to reduce the heat intrusion, a part of a current path in the current lead may be formed of a superconductor (refer to Japanese Laid-Open Patent Application JP-H07-142237 and Japanese Laid-Open Patent Application JP-2004-111581). Under the extremely-low temperature condition, an electric resistance of the superconductor in the current lead becomes small while heat resistance thereof becomes large. In this case, however, it is necessary to provide a current lead cooling unit for cooling the superconductor in the current lead, in addition to the superconducting coil cooling unit for cooling the superconducting coil. The superconductor in the current lead can be cooled without increasing heat load on the superconducting coil cooling unit.
To cool the superconducting coil rapidly to a target temperature means reduction in a time to starting-up of an operation of the superconducting coil. It is therefore important to reduce a cooling time of the superconducting coil. However, to unnecessarily increase cooling capacity of the superconducting coil cooling unit or to provide an additional superconducting coil cooling unit is not preferable, because it causes increase in costs and may be physically difficult in terms of a placement space.